Diseases + Pests — Pest of the Month: Aphids Disease of the Month: Cercospora Leaf Spot of Hydrangea

by Heather Prince

Pest of the Month: Aphids

This year it seems aphids are everywhere on everything, much to the joy of the ants farming their honeydew. There are about 50,000 different species of aphids worldwide, and most tend to be host plant specific. They feed by sucking the fluids from leaves and stems while they are expanding, often distorting their host plants. Aphids also produce sugary frass called honeydew that ants find delicious. The honeydew coats the leaves giving them a shiny, sticky appearance and often supports a secondary attack of sooty mold growth. Aphids are small and pear-shaped and can be many colors, including green, brown and yellow. They can be identified by small structures called cornicles, which look like two tailpipes on their rear ends. They usually have many generations per season, so it’s always a good idea to scout for them regularly.

Treatment:

There are many aphid natural predators including lady beetles, hover flies, lacewings, and parasitic wasps. Aphid infestations are cause for concern

Disease of the Month: Cercospora Leaf Spot of Hydrangea

Summer is for hydrangeas and as clients enjoy their plants and you maintain them, you may come across black leaf spots on the foliage of Hydrangea macrophylla, H. arborescens, and H. quercifolia. This is likely cercospora leaf spot which won’t kill the plant but may impact vigor and flowering.

when they completely cover stems and leaves, dramatically affecting plant growth. A handful here and there will be easily controlled by their predators. Small populations can be blasted with a hard stream from the hose or pruned off and disposed of. When populations are significant, use insecticidal soaps or oil products for organic control. Contact insecticides are also effective and include pyrethroids, chlorantraniliprole, acephate, and abamectin as well as a systemic treatment of imidacloprid. Make sure to apply insecticides after plants have flowered in order to avoid killing off beneficial pollinators.

Aphids

gressively spread upward. Initially, spots are purple and small with a circular shape. As spots enlarge, they often become irregular or angular in shape and develop a tan or gray center surrounded by a purple or brown border. Leaves that are severely spotted often become a yellow-green color.

Treatment:

If you have this pathogen present, sanitation is essential. Remove and destroy fallen leaves to reduce overwintering spores, avoid overhead watering of plants, and space plants to allow air movement. Clean tools between any pruning cut. Chemical control is most effective at the onset of the disease. Fungicides containing chlorothalonil or myclobutanil work best.

This pathogen overwinters on leaf debris and then spores are spread to foliage via splashing rain or overhead irrigation. You’ll find the first leafspots appear on older leaves near the plant’s base, then pro-

Additional resources:

University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php

217-333-0519

The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/

The Landscape Contractor August 2023

Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice

847-835-0972